This disclosure relates generally to electrodes and, more specifically, to methods for preparing composition, sheet comprising the composition, and electrode comprising the sheet and used in electrochemical device such as supercapacitor, fuel cell and supercapacitor desalination.
Supercapacitors are energy storage devices having high capacitance per unit mass (of the order of several tens of farads per gram (F/g) of active material to about 100 F/g of active material) and high instantaneous specific power. Supercapacitor electrosorption deionization is proposed recently as a new desalination technology to lower water treatment cost and prevent environmental pollution.
A supercapacitor comprises two identical electrodes, an electrolyte, and a separator sandwiched by the electrodes and permeable to ions of the electrolyte. Supercapacitors are categorized into different types depending on the structure of the electrodes and the nature of the electrolytes. One type of supercapacitors has an organic electrolyte and activated carbon electrodes with a large specific surface area lying in the range 1000 m2/g to 3000 m2/g, and operates electrostatically.
The activated carbon electrodes of a supercapacitor are obtained by depositing a paste sheet on a current collector. The paste is a mixture of an active carbon, a solvent, and a binder. Polytetrafluoroethylene (PTFE) is commonly used as the electrode binder.
In preparing the paste sheet, PTFE, carbon and solvents are mixed under high shear and high temperature, biaxially calendered at high temperature, extruded into the final form at high temperature, and dried at high temperature to remove the solvents. High temperatures, especially those approaching the boiling point of water, cause water lost quickly. As water is lost, the viscosity of the material rises in an uncontrolled manner, the rate of fibrillation of PTFE increases quickly, and it is very difficult to fibrillate the PTFE to a consistent level. Drying also causes water that had been incorporated into the very small pores within and around the carbon particles to be removed as vapor. It usually takes an extremely long time to rewet the carbon PTFE material and some of the originally wet internal pores of the carbon PTFE material even cannot get rewetted again.
It has been proposed to run this operation at room temperature, low shear rate and without drying. However, this method mixes all materials in one step (one-step method) and induces non-uniform mixing of PTFE and poor fibrillation of PTFE, resulting in poor electrode sheet. Furthermore, this method usually takes a relatively long time.
A need therefore exists for improved methods for preparing composition, sheet comprising the composition and electrode comprising the sheet.